1. Field of the Invention
The present invention relates to an amplifying circuit, and particularly to an amplifying circuit that can suppress spikes of an audio signal outputted by the amplifying circuit.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating an amplifying circuit 100 for outputting an audio signal according to the prior art. The amplifying circuit 100 includes an integration module 102, a comparison module 104, a triangular wave generator 106, an output module 109, a feedback resistor 111, and a low-pass filter 112. After the integration module 102 receives an input signal VIN, the integration module 102 generates and outputs a first voltage signal V1 to the comparison module 104. The comparison module 104 compares the first voltage signal V1 with a triangular wave signal VT generated by the triangular wave generator 106 to generate a comparison signal VPWM. The output module 109 generates an output signal VOUT according to the comparison signal VPWM, where the output signal VOUT is fed back to the integration module 102 through the feedback resistor 111. The low-pass filter 112 is coupled to the output module 109 for converting and filtering the output signal VOUT into an audio signal VA. A speaker 114 is coupled to the low-pass filter 112 for converting the audio signal VA into sound.
Please refer to FIG. 2A and FIG. 2B. FIG. 2A is a diagram illustrating the audio signal VA exhibiting spikes when an amplitude of the input signal VIN is increased gradually, and FIG. 2B is a diagram illustrating pulse width modulation of the output signal VOUT being stopped when the amplitude of the input signal VIN is increased gradually. As shown in FIG. 2A, when the amplitude of the input signal VIN is increased gradually, a positive half period and a negative half period of the audio signal VA may generate the spikes. Then, the spikes cause listeners to hear unpleasant noise when listening to music. In addition, as shown in FIG. 2B, the pulse width modulation of the output signal VOUT is stopped when the amplitude of the input signal VIN is increased gradually. That is to say, the output signal VOUT can stay at a high voltage or a low voltage. The output signal VOUT staying at the high voltage or the low voltage can be fed back to the integration module 102 through the feedback resistor 111, resulting in the first voltage signal V1 being saturated to the high voltage or the low voltage. Please refer to FIG. 3. FIG. 3 is a diagram illustrating the first voltage signal V1 outputted by the integration module 102 being saturated to the high voltage or the low voltage. As shown in FIG. 3, when the input signal VIN returns to a normal operation range, the first voltage signal V1 outputted by the integration module 102 needs a period of time to return to an amplitude range of the triangular wave signal VT from the high voltage or the low voltage, resulting in the audio signal VA exhibiting the spikes.